1. Field of the Invention
The invention relates to a control device and a control method for a plurality of rotating electric machines used as drive sources of a vehicle.
2. Description of Related Art
Conventionally, there are cases where a plurality of rotating electric machines are used as drive sources of a vehicle. For example, there are cases where a rotating electric machine for front-wheel drive and a rotating electric machine for rear-wheel drive are used. A torque command generated in accordance with the depression of an accelerator pedal or the like is distributed to each rotating electric machine. A command signal indicative of electric power supplied to each rotating electric machine is generated in accordance with the distributed torque command.
In addition, in the case where a direct current (DC) power source such as a battery or the like is used as the power source of the rotating electric machine, an inverter for converting DC power to alternating current (AC) power is used. The inverter performs DC/AC conversion in response to a command signal.
When the frequency of the command signal (command frequency) is low or 0 Hz, inverter lock in which a current flows to a specific switching element of the inverter for a long time period may occur. At this point, if a large current flows to the switching element, the switching element may be overheated.
The above state can occur at the time of start of a vehicle on an upward slope. In the case where an induction rotating electric machine is used, the frequency of the command signal to the rotating electric machine is the sum of a rotational frequency of a rotor and a slip frequency. When a stopped vehicle is caused to start on an upward slope, a driver releases a brake pedal and steps on an accelerator pedal. At this point, a positive torque command is outputted with depression of the accelerator pedal (the slip frequency: positive) while the vehicle slightly slides down on the upward slope with the release of the brake pedal (the rotational frequency: negative). As a result, part of all of the rotational frequency and the slip frequency cancel each other out, and the inverter lock may occur. In addition, at the time of start of the vehicle on the upward slope, there are cases where the depression of the accelerator pedal becomes relatively strong. With this, a large current flows to the switching element.
In order to reduce the overheat of the switching element, inverter protection control that reduces the current or voltage to the inverter when the flow of the large current to the switching element is detected is conventionally performed. For example, in Japanese Patent Application Publication No. 2007-259631 (JP 2007-259631 A), when the inverter lock state (a state in which an RPM becomes substantially zero) of a synchronous rotating electric machine is detected, electric power applied to the inverter is reduced. In addition, in Japanese Patent Application Publication No. 2012-228131 (JP 2012-228131 A), when the inverter lock state is detected, an inverter lock allowable time period during which the inverter lock state is allowed in accordance with the temperature of a coil of the rotating electric machine and an output torque value is calculated. Further, in Japanese Patent Application Publication No. 2005-354785 (JP 2005-354785 A), when the inverter lock state is detected, the current is distributed to the switching element on which the current is not concentrated.
Furthermore, in Japanese Patent Application Publication No. 8-289405 (JP 8-289405 A), the maximum value of the slip frequency to the induction rotating electric machine is changed in accordance with the temperature of a motor.